628 Immunity and Resistance 



been attributed to selection. Thus, native children may acquire endemic 

 diseases at an early age, with resulting death of the weakest. Conse- 

 quently, the survivors in each generation represent a selected group with 

 a resistance greater than that of the average incoming foreigner. So far 

 as malaria is concerned, some authorities believe that differences in the 

 immune status of racial groups depend primarily upon frequency of 

 infection. Whether this explanation accounts for the relatively higfi re- 

 sistance of American Negroes, perhaps even young children (32), to 

 Plasmodium vivax has been questioned. However, if there actually is such 

 a racial difference in resistance, it apparently does not extend to Pacific 

 strains of P. vivax since the susceptibility of American Negroes to Pacific 

 vivax malaria does not differ significantly from that of American whites 

 (38). Nevertheless, an apparently valid racial difference in susceptibility 

 to Plasmodium, knowlesi has been reported by Milam and Coggeshall 

 (115). Experimentally infected Negroes showed appreciably longer in- 

 cubation periods than whites and their blood remained infective to mon- 

 keys for a shorter time. 



Individual variations in natural resistance also have been reported, 

 particularly in the induction of therapeutic infections with malarial para- 

 sites. If the possibility of previous experience with the parasites can be 

 eliminated, refractory individuals presumably exhibit an effective degree 

 of natural resistance. 



The factors influencing occurrence and degree of natural resistance 

 are mostly unknown, since only a few cases have been investigated from 

 this standpoint. In some instances, body fluids of the host play an impor- 

 tant part in the fate of incoming parasites. Thus, coccidian oocysts pass 

 unchanged through the digestive tract of a naturally immune animal, 

 whereas hatching is apparently facilitated by the digestive fluids in a 

 susceptible animal (4). Occasionally, resistance may depend upon para- 

 siticidal properties of the body fluids. For example, after inoculation into 

 various cold-blooded vertebrates, Trypanosoma evansi fails to appear in 

 the blood of such animals as the eel, the serum of which destroys the 

 flagellates in vitro (104). An analogous factor presumably is responsible 

 for resistance of chickens to Plasmodium cathemerium. Immersion of 

 sporozoites in hen's blood for 30 minutes or more greatly reduces their 

 infectivity for canaries (21), which are normally susceptible to this 

 parasite. 



For individual organisms, age is an important factor in resistance. In 

 general, young animals are more easily infected and usually show more 

 severe symptoms than older animals. Trypanosoma lewisi, for instance, 

 is frequently lethal in young rats but normally produces mild and self- 

 terminating infections in adult hosts (60, 64). T. cruzi also produces lethal 

 infections in young rats and comparatively mild infections in mature ani- 

 mals (100). Comparable differences have been noted even in chick em- 



